Since the 1970's a high efficiency prime mover with renewable energy storage has been a goal of motor vehicle and distributed electric generation design to provide energy independence, conserve fossil fuels, and reduce emission of combustion products. This has led to an increased need for clean and reliable energy storage devices, which can store the power generated from clean energy sources, and make it readily available when needed in a wide range of applications. As fossil fuels are consumed more rapidly than they can be produced, an “energy crisis” has emerged and there is a widely recognized need to develop new energy technologies. Moreover, the products of combustion are both unhealthy and dangerous for the environment, while the gradual increase in temperature of the earth's atmosphere, or “greenhouse effect”, advises development of energy technology that minimizes the release of heat and greenhouse gases. Some examples of technologies that exploit natural “clean” energy sources include solar photo-voltaic panels, turbines, and geo-thermal systems. Other technologies, many focusing on motor vehicles, include recovery of vehicle deceleration and draft loss.
Energy storage of solar, wind and other intermittent sources has, in general, been dominated by batteries, which are resource intensive to manufacture, have limited number of charge cycles and may present a fire hazard. Other storage concepts under development are too expensive, hazardous or inefficient, including super capacitors, flywheels and compressed air. Renewable fuels are useful for extended unavailability of intermittent energy sources, but are in limited use, including compressed hydrogen, liquid natural gas, and bio-fuels. Hydrogen is produced inefficiently by electrolysis of water or steam reforming of methane from natural gas, which is available via the environmentally controversial fracking process. Because hydrogen is burned in inefficient converters, on-board vehicle storage is problematic and high pressures must be employed. While carbon from production of fuels may be captured for reuse or conversion to benign compounds, combustion of bio-fuels normally discharges carbon dioxide to the atmosphere.
Phase change of liquid air or nitrogen is considered to be a promising alternative storage means, finding application in electric generation and in motor vehicles. The liquid or solidified gas is referred to hereinafter as heat sink refrigerant produced by refrigerant condensation. A “liquid nitrogen economy” has been proposed [Kleppe, J. and Schneider, R., “A Nitrogen Economy”, ASEE, 1974] and some high pressure engines with phase change storage using cryogenic compression have been tested. These include a fired turbine [Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System”, Mitsubishi Tech. Review Vol. 35-3, 1998] and two fuel-less reciprocating engines [Knowlen, C. et al, “High Efficiency Energy Conversion Systems for Liquid Nitrogen Automobiles”, U. of Washington, SAE 981898, and [Ordonez, C. et al, “Cryogenic Heat Engine for Powering Zero Emission Vehicles”, ASME Intl. Mech. Engineering Congress & Expo., 2001]. More recently, phase change storage is gaining acceptance in the United Kingdom as indicated by an operating 300 kW pilot plant and a fueless liquid nitrogen engine for compact urban vehicles [Center for Low Carbon Futures, “Liquid Air in the Energy and Transport Systems”, ISBN:978-0-9575872-2-9, 2013]. In these prime movers, low compression work is attained by incompressible working fluid, which may include combustion air. Consumption of refrigerant is excessive in these high pressure engines (40 to 80 bar), which are not optimized for thermodynamic cycle or pressure ratio, nor supplemented by recovered energy. Improved engines have been proposed including a closed Brayton cycle with ambient source and quasi-isentropic cryo-compression sink [Ordonez, C., “Liquid Nitrogen Fueled, Closed Brayton Cycle Cryogenic Heat Engine”, Energy Conversion & Management 41, 2000], and an open Brayton cycle with over ambient source and quasi-isothermal cryo-compression sink [Kaufman, J., “U.S. Pat. No. 7,854,278 B2”, 2010]. These two cryogenic compression concepts would economize refrigerant consumption and profoundly impact design and production capacity of refrigerant condensation facilities.
Initial efforts to provide refrigerant are focused on available sources including low cost off-peak electric grid power and the heat of evaporation of liquid natural gas (lng), which is normally not recovered during distribution. While selection of these energy sources is based on availability and economics, inherent disadvantages include transmission loss, transport of the refrigerant and perpetuation of the environmental downside of centralized fossil fuel and nuclear use.
It is recognized that energy to drive refrigerant condensation must ultimately derive from renewable sources and be universally available for smaller scale distributed use in both stationary and motor vehicle application. As expected, there are also issues with the use of renewables, primarily unavailability of solar, wind and geo-thermal, and inefficiency of energy conversion components in all recovery processes. It is important to minimize refrigerant consumption, especially in motor vehicle use. Towards this end, captured vehicle energy will reduce refrigerant as well as fuel consumption. Vehicle energy recovery modes of solar, deceleration and draft loss also have issues. Solar recovery is limited by available capture area. Deceleration recovery is a developed technology, but limited by inherent compression braking with reciprocating engines. Capture of vehicle draft loss [Kaufman, J., “U.S. Pat. No. 7,854,278 B2”, 2010] can be substantial at highway speeds, but is limited as for stationary wind, described above.
Following is a description of prior art components for phase change storage for providing refrigerant to cryo-compression engines:
Refrigerant condensers may liquefy or solidify a gas in special applications by transfer of heat to a lower temperature sink, as for example a vapor discharge sink for use during distribution of lng. More commonly, however, condensation is by various vapor-compression cycles. Gas is expanded by venting into a chamber while expansion causes a temperature drop and the pressurized gas entering the expander is further cooled by counter-flow heat exchange to the vented portion of expanded gas. Heat of compression in these machines is removed by cooling to ambient in an attached heat exchanger. Large central vapor-compression liquefiers are attaining efficiency of about 50%, however this requires complex expensive equipment with features such as pre-cooling and multiple expansion of refrigerant. To attain high efficiency in smaller applications other condensation concepts are under development. These include magnetic refrigeration, [Matsumoto, K. et al, “Magnetic Refrigerator for Hydrogen Liquefaction, J. of Physics: Conf. Series 150, 2009, and thermo-acoustic refrigeration [Wollan, J. et al, “Development of a Thermoacoustic Natural Gas Liquefier”, Los Alamos Natl. Lab., LA-UR-02-1623, AlChE, 2002].
Several renewable energy compressor drives for vapor-compression liquefiers have been proposed for stationary application, including solar, wind and fuel fired. On-board motor vehicle refrigerant condensation is considered to be impractical due to low condensing efficiency and limited storage capacity.
Photo-voltaic panels convert light to electricity using a semi-conducting material such as silicon that exhibits the photo-voltaic effect. Natural convection cooling limits performance degradation of the cells. Performance increases from about 20 to 40% with concentrated radiation by mirror or lens, however, a forced circulation system is required to limit temperature rise. Other disadvantages include the economics of advanced cell technology including photo-voltaic material, complexity of concentrating, tracking and multi-junction/material systems, and losses due to battery charge/discharge and fouling of panel surfaces. Solar thermal systems require complex engine driven systems and performance is limited to an efficiency of about 20% due to low heat source temperature.
While free wind may be insufficient in most locations to provide viable liquefier drive, amplification of wind due to the presence of structures offers distributed energy recovery potential. Building amplified wind [Kaufman, J., “U.S. Pat. No. 9,395,118”, 2016] operates by a self aspirating mechanism, in which wind impacting a structure suctions a smaller recovery flow through a turbine-generator. Output of this mechanism is limited by flow through the recovery turbIne, estimated at 25% of impact flow.
Finally, refrigerant liquefiers may be driven by renewable fuel fired combustion engines. In addition to low engine efficiency, issues associated with state-of-art fuels to drive refrigerant condensation include high pressure requirement for on-board storage of gaseous fuel, cost of fuel liquefaction and carbon capture during fuel synthesis and combustion. Hydrogen is an exception, as it burns without carbon release, however, it has containment issues including, corrosiveness, high reactivity, gas storage pressure and liquefaction cost.